TechArchive

Original article date: February 1999

It’s a myth that electronic weighing systems are inappropriate for use in hazardous areas says CHARLES WARREN Industrial Product and Marketing Manager of Avery Berkel.

With increased concerns for the health and safety of personnel and plant more and more areas previously classified as “safe” are now being reclassified under European safety regulations as hazardous. The use of instrumentation in so-called hazardous areas is strictly regulated by industry-standard Codes of Practice.

Tracking the transition to fast high accuracy digital weighing by industry over the last decade or so it is apparent that the process industry has largely relied on mechanical equipment for hazardous applications. There has been no ideal safe digital weighing solution suitable for major parts of the industry.

Where electronic weighing has been applied in hazardous areas several methods of protection have emerged (see panel). Depending on the application each varies in effectiveness and some methods can present serious drawbacks. All involve extensive installation work and continue to incur high costs for on-going servicing and maintenance.

Intrinsically safe weighing equipment

Intrinsic safety – an energy limitation approach – is the preferred method of protection against explosion. It is the only technique permitted for use in Zone 0 areas because the intrinsically-safe circuit produces no sparks or thermal effects capable of causing ignition.

Avery Berkel’s latest intrinsically safe hazardous area instrumentation offers a typical accuracy of +/-1 part in 6000 and has been developed to suit hazardous zones 1 and 2. Linked to platform scales load cell weighers or vessel weighing systems this instrumentation uses low power technology for safe weighing in hazardous areas. It is approved by DEMKO (The Danish board for the approval of electrical equipment) and SIRA to CENELEC standards.

This indicator family may be connected to a wide range of CENELEC-approved load cell weighing platforms and systems providing a standalone weighing or filling installation without enclosures gas/air supplies or cable communications back to the safe area. Units are made from stainless steel treated by pickling and sometimes by electropolishing. This decreases the risk of cross-contamination and helps to ensure the scale’s durability.

As there is sufficient energy in the instrument’s circuits to cause an incendiary spark installation and maintenance may be carried out enclosures may be opened and printed circuits may be interchanged all in the hazardous area and without stopping the process. Indicators may be AC or battery-powered and include a mobile weigher.

Avery Berkel has also launched the RDREx range of rugged electronic machines for heavy-duty hazardous area weighing challenging the notion that only mechanical machines are suitable for cost-effective weighing in explosive environments.

The above area classification deals only with risks associated with inflammable gases and vapours and by implication flammable mists. It does not deal with dusts for which an area classification code is being developed.

An area that is not classified Zone 0. 1 or 2 is deemed to be non-hazardous and normal electrical techniques generally apply with the use of normal industrial electrical equipment.

BS EN 600769-10:1996 is the latest publication dealing with the classification of hazardous areas for gases and vapours.

Ingress Protection (IP) Codes

(BS EN 60529:1992)

As well as the Hazardous Area safety rating process control instrumentation is also given a coding for the degree of “hermeticity” to be expected when used with common process chemicals and solvents – there is often a requirement for these to be hose-proof and dust-tight too.

Traditional methods of protection for weighing equipment

Flameproofing

• Known as explosion-proofing in the USA this technique involves housing standard instrumentation in a “bomb-proof” enclosure. Small explosions which may be caused by the electrical equipment inside allowed to take place within the enclosure under carefully controlled conditions.
• Flameproofing is permitted for Zone 1 and Zone 2 areas only and the safety of the equipment is crucially reliant on careful maintenance.
• Equipment is heavy and cumbersome to install and allows only a limited view of the weight display through very thick safety glass and very limited access to the glass.

Sand filling

• Sand or quartz filling – thus limiting the spread of ignition – is limited to Zone 2 applications.
• Has found popularity within some European countries.
• Safety of equipment is dependent on stringent maintenance procedures.
• Involves return-to-factory filling of sand/quartz so downtime is a problem.

Gas-purged

• Gas-purged enclosures house the weighing instrumentation within a pressurised box.
• Safety of the system depends on supplies of clean air or inert gas continuously feeding through the enclosure backed up by good fail-safe start-up and shut-down procedures.
• Equipment is bulky and difficult to install with costly runs of cabling and pipes for gas or air supplies.
• Gas or air is expensive to supply.
• Weighing is impeded by difficult access to instrumentation controls.

Barrier methods

• Consists of load cell equipment situated in the hazardous area with cabling back to standard instrumentation in the safe area connected via a Zener barrier box.
• Work by preventing large amounts of energy getting into the hazardous area.
• Permitted in Zones 1 and 1 only.
• As instrumentation is remote from the weighing operation it is customary to install a special weight display unit next to the load cell platform necessitating yet more barriered circuits back to the hazardous area (or the use of flameproof equipment).
• Installation is complex and expensive – often involving long runs of cabling.
• Weighing resolution is degraded by the presence of barriers in the system.
• Control of the instrument is very limited from within the hazardous area.

• Avery Berkel
• Charles Warren
• 0121 558 1112

February 1999